Synthetic Lubricating Oil

ABSTRACT

The object of the present invention is to provide a synthetic lubrication oil which shows low viscosity, is excellent in viscosity properties at high temperature and shows stable lubricating properties in a wide range of temperature. The synthetic lubricating oil is one comprising an ionic liquid containing an organic cation selected from the group consisting of an imidazolium cation, a pyridinium cation, a quaternary ammonium cation and a quaternary phosphonium cation and a bis(fluorosulfonyl)imide anion, and one comprising an ionic liquid composition which comprises an ionic liquid (A) containing a 1-ethyl-3-methylimidazolium cation and an ionic liquid (B1) containing a 1-methyl-3-propylimidazolium cation and/or an ionic liquid (B2) containing a 1-methyl-3-isopropylimidazolium cation.

RELATED APPLICATION

This application is a U.S. national phase application under 35 U.S.C.§371 of International Application No. PCT/JP2006/322477 filed Nov. 10,2006 which claims priority of Japanese Patent Application No.2005-328695 filed Nov. 14, 2005 and Japanese Patent Application No.2006-240058 filed Sep. 5, 2006.

TECHNICAL FIELD

The present invention relates to a synthetic lubricating oil comprisingan ionic liquid selected from an organic cation and abis(fluorosulfonyl)imide anion, and to a synthetic lubricating oilcomprising an ionic liquid composition which is a mixture of two or moreionic liquids containing a 1,3-substituted imidazolium cation.

BACKGROUND ART

As a lubricating oil used in a machinery installment, a powertransmission device, a metallurgical processing oil, grease and thelike, base oil selected from a poly α-olefin, a diester, a polyol ester,silicone and the like as the best ingredient has so far been used underoptionally mixing with a suitable additive. However, those lubricatingoils are accompanied with a danger of flash ignition or evaporationunder specific conditions such as high temperature and highly vacuumpressure, and thus more suitable lubricating oil has been desired.Further, in accordance with high functionality and high efficacy of adevice, a lubricating oil having more excellent anti-oxidation property,anti-evaporation property and long-released excellent lubricatingproperty has been desired.

As means for solving the above problems, it is reported in R. A. Reichet al., Journal of the Society of Tribologists and LubricationEngineers, July 2003, p. 16 to 21, for instance, that a compoundcomprising a combination of an organic cation and an inorganic anion(ionic liquid, melted salt at normal temperature) can be adopted aslubricating oil, and it has recently been known that the ionic liquid ispossibly usable as a material of a lubricating oil because the ionicliquid is excellent in non-volatility, stability in a wide temperaturerange and fire-resistance, and also it has a high viscosity index andsufficient properties of a coefficient of friction and an abrasion tracediameter which are required for lubricating oil.

However, many of the ionic liquids have generally show high viscosity,and thus it is necessary for practical use as a lubricating oil to findout an ionic liquid having low viscosity, and under the situation,further development has been continued.

And, in a Japanese Patent Publication No. 2005-89667, an ionic liquidcontaining bis(trifluoromethanesulfonyl)imide anion as an anion ismentioned as an ionic liquid having low viscosity. However, it has beensaid that the ionic liquid comprising this combination of an anion isnot sufficient in reduction of viscosity.

DISCLOSURE OF INVENTION

Under the situation, the object of the present invention is to provide asynthetic lubricating oil having a stable lubricating property in a widetemperature range.

Thus, the present inventors have made extensive study under consideringthe above situation to find surprisingly that an ionic liquid havingremarkably low viscosity and excellent viscosity stability can beobtained by using a bis(fluorosulfonyl)imide anion as an anion seed inan ionic liquid using an organic cation selected from an imidazoliumcation, a pyridinium cation, a quaternary ammonium cation and aquaternary phosphonium cation and this ionic liquid can suitably be usedas a synthetic lubricating oil.

Further, it is found that an ionic liquid composition having a lowviscosity and excellent viscosity stability and moreover having a lowermelting point compared with a single use can be obtained by mixing anionic liquid with an ionic liquid containing an imidazolium cationhaving a specific alkyl group of relatively short alkyl chain and at thesame time by incorporating therein two or more kind of compounds havingan eutectic point, and consequently the present invention has beencompleted.

Namely, the present invention relates to a synthetic lubricating oilcomprising an ionic liquid composition which comprises an ionic liquid(A) containing an organic cation selected from the group consisting ofan imidazolium cation, a pyridinium cation, a quaternary ammonium cationand a quaternary phosphonium cation and a bis(fluorosulfonyl)imide anionand an ionic liquid (B1) containing a 1-ethyl-3-methylimidazoliumcation, and an ionic liquid (B2) containing a1-methyl-3-propylimidazolium cation and/or a1-methyl-3-isopropylimidazolium cation.

The ionic liquid and the ionic liquid composition used in the presentinvention are useful as lubricating oil which is excellent inanti-abrasion property and further has stable flowability, and viscositycharacteristics required for lubricating oil, and still further hasstable lubricating properties in a wide temperature range and a wideusing condition.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, the present invention is explained in details.

In the following, the ionic liquid of the present invention means anionic substance which is in melted state at normal temperature (25° C.).

The present invention relates to a synthetic lubricating oil comprisingan ionic liquid composition which comprises an organic cation selectedfrom the group consisting of an imidazolium cation, a pyridinium cation,a quaternary ammonium cation and a quaternary phosphonium cation and abis(fluorosulfonyl)imide anion and to a synthetic lubricating oilcomprising an ionic liquid (A) containing a 1-ethyl-3-methylimidazoliumcation, and an ionic liquid (B1) containing a1-methyl-3-propylimidazolium cation and/or an ionic liquid (B2)containing a 1-methyl-3-isopropylimidazolium cation.

Explanation is given first on the synthetic lubricating oil comprisingan ionic liquid which comprises an organic cation selected from thegroup consisting of an imidazolium cation, a pyridinium cation, aquaternary ammonium cation and a quaternary phosphonium cation, and abis(fluorosulfonyl)imide anion.

The organic cation used in the present invention is exemplified by animidazolium cation, a pyridinium cation, a quaternary ammonium cationand a quaternary phosphonium cation. By combining the organic cationwith a bis(fluorosulfonyl)imide anion, viscosity of the ionic liquid asa synthetic lubricating oil can be reduced to a great extent.

The imidazolium cation is not specifically limited and can beexemplified by one having a structure shown by the following generalformula (I).

(in the formula (I), the substituents R¹ to R⁵ are each independently ahydrogen atom, a halogen atom, a straight chained or branched alkylgroup, an alkenyl group, an alkinyl group, an alkoxyl group or an acylgroup, which has 1 to 16 carbon atoms, or an amide group, a cyano group,a nitro group, or an amino group, and the alkyl group, the alkenylgroup, the alkinyl group, the alkoxyl group and the acyl group maycontain a hetero atom selected from N, S and O, and further may containa conjugate or independent double bond or triple bond.).

In a case where the substituents R¹ to R⁵ are an alkyl group, an alkenylgroup, an alkinyl group, an alkoxyl group or an acyl group, a carbonatom number thereof is preferably 1 to 16, particularly preferably 1 to12, and still particularly preferably 1 to 6. Those substituents may bestraight chained or branched, and a carbon atom number over the abovemaximum value is not preferable because of trend of viscosity increaseby intermolecular interaction on side chains.

The above alkyl group, alkenyl group, alkinyl group, alkoxyl group andacyl group may contain a hetero atom selected from N, S and O, and thenumber of the hetero atom to be contained is not specifically limited.Further, they may contain a conjugate or independent double bond ortriple bond, and the number of those unsaturated bonds is notspecifically limited.

Those alkyl groups are specifically exemplified by a methyl group, anethyl group, a propyl group, an isopropyl group, a butyl group, anisobutyl group, a secondary butyl group, a tertiary butyl group, apentyl group, a hexyl group, a cyclopropyl group, a cyclopentyl group, acyclohexyl group, etc. The alkenyl group is exemplified by a vinylgroup, an allyl group, an 1-propenyl group, an isopropenyl group, a2-butenyl group, an 1,3-butadienyl group, a 2-pentenyl group, a2-hexenyl group, etc. Further, the alkinyl group is exemplified by anethynyl group, an 1-propinyl group, a 2-propinyl group, etc., and thealkoxyl group is exemplified by a methoxy group, an ethoxy group, ann-propoxy group, an isopropoxy group, a t-butoxy group, etc., the acylgroup is exemplified by an acetyl group, a propionyl group, a butylylgroup, a benzoyl group, etc., and the amino group is exemplified by anN,N-dimethylamino group, an N,N-diethylamino group, etc. From aviewpoint of industrial use, easy decomposition by enzymes and increasedbiodegrability are valuable, and thus an alkoxyl group, an acyl group,an amide group, a cyano group, a nitro group, an amino group, etc. canbe mentioned.

As the imidazolium cation shown by the above formula (I),1,3-substituted imidazolium cation, and 1,2,3-substituted imidazoliumcation is preferably used from a viewpoint of easy synthesis. Thesubstituent in the derivatives may be same or different, and asubstituent which may contain a multiple bond or a branched chain ispreferable.

The above substituent is same with one in the above general formula (I),and one optionally selected therefrom is used.

Further, in the present invention, a pyridinium cation, a quaternaryammonium cation and a quaternary phosphonium cation are mentioned aswell as the above imidazolium cation, and the pyridinium cation isexemplified by a pyridinium cation substituted by an alkyl group of 1 to16 carbon atoms such as N-methylpyridinium, N-ethylpyridinium,N-butylpyridinium and N-propylpyridinium.

The quaternary ammonium cation is exemplified by an ammonium cationsubstituted by an alkyl group of 1 to 16 carbon atoms such astetramethylammonium, tetraethylammonium and tetrabutylammonium.

The quaternary phosphonium cation is exemplified by a phosphonium cationsubstituted by an alkyl group of 1 to 16 carbon atoms such astetramethylphosphonium, tetraethylphosphonium and tetrabutylphosphonium.

And, in the present invention, the anion used as an anion partconstituting the ionic liquid together with the above organic cation isa bis(fluorosulfonyl)imide anion, and by using this anion, an ionicliquid having remarkably low viscosity and excellent viscosity stabilitycan be obtained.

A method for producing the ionic liquid of the present invention is notspecifically limited, and a conventional method such as an ion exchangemethod or a metathesis reaction can be applied. For instance, the ionicliquid can be obtained by an anion exchange reaction using a halogenatedsalt of an organic cation to be used and an alkaline metal salt of abis(fluorosulfonyl)imide anion. The halogen in the halogenated salt isexemplified by chlorine or bromine. The alkaline metal in the alkalinemetal salt is exemplified by sodium, potassium, etc.

Amounts of the halogenated salt of the organic cation and the alkalinemetal salt of a bis(fluorosulfonyl)imide anion to be used in the abovereaction are not specifically limited, and 0.5 to 2 equivalents, stillpreferably 0.8 to 1.2 equivalent of the alkaline metal salt ofbis(fluorosulfonyl)imide anion relative to the halogenated salt of theorganic cation is preferable. In a case of over the above range,economical effect tends to be lowered because the amount over the rangedoes not give influence upon a reaction yield, and in a case of lessthan the range, on the other hand, a large amount of non-reactedstarting material remains to bring about tendency of lowering a reactionyield.

The synthetic lubricating oil containing the ionic liquid comprising theabove organic cation and bis(fluorosulfonyl)imide anion may contain, aswell as the above ionic liquid, a base oil for a lubrication oil, whichhas been conventionally used, and further optionally other additivessuch as an anti-rusting agent and a flow-point depressant can be used.An amount of the additives is not specifically limited so far as theeffect of the present invention is not disturbed, and it is preferably0.001 to 50% by weight relative to the ionic liquid in order to utilizethe characteristics owned by the ionic liquid itself.

Then, explanation is given on the synthetic lubricating oil comprisingthe ionic liquid composition which comprises the ionic liquid (A)containing a 1-ethyl-3-methylimidazolium cation and the ionic liquid(B1) containing a 1-methyl-3-propylimidazolium cation and/or the ionicliquid (B2) containing a 1-methyl-3-iospropylimidazolium cation.

Those ionic liquid compositions have been found as a result of studyingcombinations of ionic liquids containing various kind of imidazoliumcation. The compositions comprising combinations of those ionic liquidshave low viscosity and excellent viscosity stability, and further havelower viscosity than each of the ionic liquids themselves, and thus ithas been found that those compositions can be used as lubricating oil ata low temperature in a wider range.

The anion parts of the ionic liquids (A), (B1) and (B2) used in thepresent invention are not specifically limited, and anions so far usedin conventional ionic liquids can be used. Among them, use of ahydrophobic organic anion or inorganic anion is preferable from aviewpoint of keeping hydrophobicity, and as the hydrophobic organicanion, a bis(trifluoromethanesulfonyl)imide anion, a(fluorosulfonyl)(trifluoromethanesulfonyl)imide anion, a(trifluoroacetyl) (trifluoromethanesulfonyl)imide anion, etc. arepreferably used, and as the hydrophobic inorganic anion, ahexafluorophosphate anion, a bis(fluorosulfonyl)imide anion, etc. arepreferably used, among which a bis(fluorosulfonyl)imide anion is mostpreferably used.

The anion parts of each ionic liquid constituting the ionic liquidcomposition of the present invention may be same with or different fromeach other, and the same anion parts are particularly desirable becauseof easier production compared with different anion parts.

A mixing ratio (weight ratio) of the ionic liquid (A) to the ionicliquid (B1) and/or the ionic liquid (B2) to be used in the presentinvention can be selected suitably according to the desired viscosityand low temperature for its use, and generally, a ratio (A):(B1) and/or(B2)=8:2 to 2:8 is preferable. Over the maximum in the ionic liquid (A)tends to cause increase of a melting point, and less than the rangetends to cause increase of viscosity.

Particularly in a case of incorporating the ionic liquid (A) and theionic liquid (B1), its mixing ratio (weight ratio) of (A):(B1)=7:3 to2:8 is preferable, and in a case of incorporating the ionic liquid (A)and the ionic liquid (B2), its mixing ratio (weight ratio) of(A):(B2)=8:2 to 6:4 is preferable. Over the maximum in the ionic liquid(A) tends to cause increase of a melting point, and less than the rangetends to cause increase of viscosity.

In the ionic liquid composition of the present invention, it is possibleto co-use the ionic liquid (A) with (B1), co-use the ionic liquid (A)with (B2) and to co-use of the ionic liquid (A) with (B1) and (B2), andalso it is possible to co-use an optional ionic liquid (C) other thanthe ionic liquids (A), (B1) and (B2) in a range not inhibiting theeffect of the present invention. The optional ionic liquid (C) isexemplified by an ionic liquid containing a 1-allyl-3-alkylimidazoliumcation, an ionic liquid containing a 1,3-diallylimidazolium cation, anionic liquid containing a 1,3-dimethylimidazolium cation, etc., but notlimited thereto.

An amount of the ionic liquid (C) is, in general, preferably 0 to 20% byweight, more preferably 0 to 10% by weight.

A method for producing the ionic liquid (A), (B1), (B2) and (C) is notspecifically limited, and a conventional method such as an anionexchange method, an acid ester method, a neutralizing method, etc. canbe applied. For instance, they can be produced by alkylating with theuse of N-alkyl imidazole and an alkylating agent such as an alkylhalide, and then conducting an anion exchange reaction using an alkylmetal salt of bis(fluorosulfonyl)imide.

The synthetic lubricating oil containing the ionic liquid compositionmay contain a conventional base oil of a lubricating oil as well as theabove ionic liquid component, and further optionally other additivessuch as an anti-rusting agent and a flow-point depressant can be used.An amount of the additives to be used is not specifically limited so faras not disturbing the effect of the present invention, and preferably0.001 to 50% by weight relative to the above ionic liquid in order toutilize the characteristics owned by the ionic liquid itself.

In this way as above, the synthetic lubricating oil comprising the ionicliquid or the ionic liquid composition of the present invention can beproduced.

Viscosity of the synthetic lubricating oil of the present invention is,in general, preferably 30 mPa·s or lower at 25° C., still preferably 20mPa·s or lower. Over the maximum of viscosity is not preferable becauseenergy loss is caused by viscosity of the lubricating oil itself, andthe minimum of the viscosity is generally 2 mPa·s, and less than theminimum tends to fly easily due to low viscosity.

Further, the kinematic viscosity of the synthetic lubricating oil of thepresent invention is preferably 2 to 20 mm²/sec at 40° C., stillpreferably 4 to 10 mm²/sec, and the kinematic viscosity at 100° C. ispreferably 1 to 13 mm²/sec, still preferably 2 to 7 mm²/sec. Over themaximum in each temperature tends to cause energy loss by viscosity ofthe lubricating oil itself, and less than the minimum tends to flyeasily due to low viscosity.

Viscosity index of the synthetic lubricating oil is preferably 180 ormore, still preferably 200 or more, particularly preferably 220 or more.The maximum viscosity index is generally 700. Herein, viscosity indexmeans an index showing a relation of temperature and viscosity, andcalculated by a method stipulated in JIS K2283 (a test method forkinematic viscosity of crude oil and petroleum products and a viscosityindex calculation method of petroleum products).

Higher viscosity index means less change by temperature, and thus meansthat the product is excellent as a lubricating oil.

As the objects to be applied by the lubricating oil, there can be imagedthose wherein high absolute viscosity is important and those whereinother properties such as contact angle to a metal is considered to bemore important than absolute viscosity. In such cases, upon necessity,the properties are controlled by selecting the organic cation among animidazolium cation, a pyridinium cation, a quaternary ammonium cationand a quaternary phosphonium cation and further optionally changing thesubstituent. Also in this case, the above viscosity index is a propertyconsidered to be important. Less than the minimum viscosity index tendsto bring about too much high changing degree of viscosity bytemperature.

The synthetic lubricating oil of the present invention has excellentviscosity properties at low viscosity and high temperature, and isexcellent in non-volatility, heat stability and other properties, andthus it has possibility of using in a wide range as lubricating oil formachinery installments such as automobiles and electrical products,power transmission devices and precision machines, as metallurgicalprocessing oil, as lubricating oil under special conditions, and thelike.

EXAMPLE

In the following, the present invention is further explained referringto Examples, but the present invention is not limited to the followingExamples unless it is beyond its gist. In Examples, “part” and “%” meanon the basis of weight unless otherwise stated. Viscosity was obtainedby the following method.

Conditions for measuring viscosity are as follows.

Machine: AR-1000 type rotary rheometer (TA Instruments).Measuring method: The machine was set forth at 25° C., and 0.6 ml samplewas put on a sample desk, and a cone was set forth, and viscosity uponrotating the cone at predetermined force (20 Pa) was measured.

The kinematic viscosity at 40° C. and 100° C. was obtained by measuringviscosity at each of the above temperature using the above machine andmeasuring method and calculating from thus obtained viscosity. Further,the viscosity index was obtained by calculating from the kinematicviscosities.

Synthesis Example 1

In a flask equipped with a refluxing column were charged 19.1 g (0.10mol) of 1-ethyl-3-methylimidazolium bromide and 20 ml of water, andfurther 21.9 g (0.10 mol) of potassium salt of bis(fluorosulfonyl)imide,followed by stirring at 40° C. for 5 hours. After the reaction, 80 ml ofmethylene chloride was added, followed by well stirring andfractionating. The methylene chloride layer was washed five times eachwith 40 ml of water, and the methylene chloride layer is concentratedunder reduced pressure to give 23.8 g (0.08 mol, yield: 81.8%) of1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide.

Synthesis Example 2

The same procedure as the above Synthesis example 1 excepting using 20.5g of 1-methyl-3-propylimidazolium bromide in place of 19.1 g of1-ethyl-3-methylimidazolium bromide was conducted to give 27.5 g (0.09mol, yield: 90.1%) of 1-methyl-3-propylimidazoliumbis(fluorosulfonyl)imide. Viscosity of this compound was 29 mPa·s (25°C.).

Synthesis Example 3

The same procedure as the above Synthesis example 1 excepting using 20.5g of 1-methyl-3-isopropylimidazolium bromide in place of 19.1 g of1-ethyl-3-methylimidazolium bromide was conducted to give 27.1 g (0.09mol, yield: 88.8%) of 1-methyl-3-isopropylimidazoliumbis(fluorosulfonyl) imide. Viscosity of this compound was 27 mPa·s (25°C.).

Example 1

Result of measuring various properties of 1-ethyl-3-methylimidazoliumbis(fluorosulfonyl)imide in the Synthesis example 1 is shown in Table 1.

Comparative Example 1

11.0 Grams (60.6 mmol) of 1-ethyl-3-methylimidazolium bromide obtainedby the same method as Example 1 was reacted with 20.3 g (63.7 mmol) ofpotassium salt of bis(trifluoromethanesulfonyl)imide anion in 20 g ofwater-methylene chloride at 40° C. for 4 hours, and an aqueous layer wasfractionated by a separating funnel and discarded, and an organic layerwas washed with water, followed by drying under reduced pressure to give22.5 g (57.6 mmol, yield: 95.0%) of 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide.

Measurement of properties of the obtained 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide was conducted by the same manner asthe Example 1. The result is shown in Table 1.

Comparative Example 2 Polyα-Olefin

Properties were measured by the same manner as the Example 1 usingpolyα-olefin (SYNFLUID 801 (Chevron phillips). The result is shown inTable 1.

Comparative Example 3 Diester

Properties were measured by the same manner as the Example 1 usingdiester (dioctyl adipate: Plasthall DOA (The C.P. Hall)). The result isshown in Table 1.

Comparative Example 4 Polyol Ester

Properties were measured by the same manner as the Example 1 usingpolyol ester (polyester (trivalent): KAOLUBE 190 (Kao Corporation)). Theresult is shown in Table 1.

Comparative Example 5 Liquid Paraffin

Properties were measured by the same manner as the Example 1 usingliquid paraffin (Cosmo Neutral 150: (Cosmo Oil Lubricants co LTD)). Theresult is shown in Table 1.

TABLE 1 Kinematic Kinematic Viscosity viscosity viscosity (25° C., (40°C., (100° C., Viscosity mPa · s) mm²/s) mm²/s) index Ex. 1 19 9.4 4.9623 Com. Ex. 1 28 12 3.7 206 Com. Ex. 2 62 46 7.8 130 Com. Ex. 3 36 255.2 140 Com. Ex. 4 65 48 9.7 151 Com. Ex. 5 44 33 5.6 108

Examples 2 to 7

Using 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide in theSynthesis example 1 as the ionic liquid (A) and1-methyl-3-propylimidazolium bis(fluorosulfonyl)imide in the Synthesisexample 2 as the ionic liquid (B1), ionic liquid compositions of theformulations in Table 2 were produced. Result of measuring propertiesthereon is shown in Table 2.

TABLE 2 Ionic Ionic liquid liquid Kinematic Kinematic (A) (B1) Viscosityviscosity viscosity EMIFSI MPIFSI (25° C., (40° C., (100° C., Viscosity(%) (%) mPa · s) mm²/s) mm²/s) index Ex. 2 80 20 20 10.3 4.7 503 Ex. 370 30 21 10.7 4.7 473 Ex. 4 60 40 22 11.1 4.8 463 Ex. 5 50 50 22 11.54.8 438 Ex. 6 40 60 23 11.8 4.9 437 Ex. 7 20 80 25 12.2 5.4 490EMIFSI: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imideMPIFSI: 1-methyl-3-propylimidazolium bis(fluorosulfonyl)imide

Examples 8 and 9

Using 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide in theSynthesis example 1 as the ionic liquid (A) and1-methyl-3-isopropylimidazolium bis(fluorosulfonyl)imide in theSynthesis example 3 as the ionic liquid (B2), ionic liquid compositionswere produced. Result of measuring properties thereon is shown in Table3.

TABLE 3 Ionic Ionic liquid liquid Kinematic Kinematic (A) (B2) Viscosityviscosity viscosity Vis- EMIFSI MiPIFSI (25° C., (40° C., (100° C.,cosity (%) (%) mPa · s) mm²/s) mm²/s) index Ex. 8 80 20 20 10.3 4.7 503Ex. 9 60 40 21 10.9 4.8 477 EMIFSI: 1-ethyl-3-methylimidazoliumbis(fluorosulfonyl)imide MiPIFSI: 1-methyl-3-isopropylimidazoliumbis(fluorosulfonyl)imide

As understood from results of Table 1 to Table 3, the syntheticlubricating oil of the present invention, which comprises the specificorganic cation and bis(fluorosulfonyl)imide anion and the syntheticlubricating oil containing the ionic liquid composition comprising amixture of two or more ionic liquids containing 1,3-substitutedimidazolium cation has lower viscosity and higher viscosity indexcompared with an ionic liquid comprising a bis(trifluoromethylsulfonyl)imide anion which has been known as being low viscosity, andother conventional base oil, and thus they are remarkably excellent inproperties as a synthetic lubricating oil.

Further, by making the cation easily degradable by introducing asubstituent for increasing biodegradability, the remaining anion becomesan inorganic compound and thus no attention to biodegradability of theanion is necessary, and consequently the oil becomes suitable also fromenvironmental point of view.

INDUSTRIAL APPLICABILITY

The synthetic lubricating oil of the present invention is suitable as alubricating oil for machinery installments such as automobiles, marinestructures and electrical products, a lubricating oil for powertransmission devices and precision machines, metallurgical processingoil, a lubricating oil under special conditions, and the like.

1. A synthetic lubricating oil comprising an ionic liquid containing anorganic cation selected from the group consisting of an imidazoliumcation, a pyridinium cation, a quaternary ammonium cation and aquaternary phosphonium cation and a bis(fluorosulfonyl)imide anion. 2.The synthetic lubricating oil of claim 1, wherein the organic cation isan imidazolium cation.
 3. The synthetic lubricating oil of claim 2,wherein the imidazolium cation is a 1,3-disubstituted imidazolium cationin which the substituents on the position-1 and the position-3 may besame or different, or a 1,2,3-trisubstituted imidazolium cation in whichthe substituents on the position-1, the position-2 and the position-3may be same or different.
 4. A synthetic lubricating oil comprising anionic liquid composition comprising an ionic liquid (A) containing a1-ethyl-3-methylimidazolium cation and an ionic liquid (B1) containing a1-methyl-3-propylimidazolium cation and/or an ionic liquid (B2)containing a 1-methyl-3-isopropylimidazolium cation.
 5. The syntheticlubricating oil of claim 4, wherein an anion part of the ionic liquid(A) and an anion part of the ionic liquid (B1) and/or the ionic liquid(B2) are same.
 6. The synthetic lubricating oil of claim 5, wherein theanion parts of the ionic liquid (A) and the ionic liquid (B1) and/or theionic liquid (B2) are bis(fluorosulfonyl)imide or(fluorosulfonyl)(trifluoromethanesulfonyl)imide.
 7. The syntheticlubricating oil of claim 6, wherein a mixing ratio (weight ratio) of theionic liquid (A) to the ionic liquid (B1) and/or the ionic liquid (B2)is (A):(B1) and/or (B2)=8:2 to 2:8.
 8. The synthetic lubricating oil ofclaim 4, wherein the anion parts of the ionic liquid (A) and the ionicliquid (B1) and/or the ionic liquid (B2) are bis(fluorosulfonyl)imide or(fluorosulfonyl)(trifluoromethanesulfonyl)imide.
 9. The syntheticlubricating oil of claim 4, wherein a mixing ratio (weight ratio) of theionic liquid (A) to the ionic liquid (B1) and/or the ionic liquid (B2)is (A):(B1) and/or (B2)=8:2 to 2:8.
 10. The synthetic lubricating oil ofclaim 5, wherein a mixing ratio (weight ratio) of the ionic liquid (A)to the ionic liquid (B1) and/or the ionic liquid (B2) is (A):(B1) and/or(B2)=8:2 to 2:8.